1. Field of the Invention
This invention is related to a discharge lamp lighting device which is used as the light source for an automobile headlight.
2. Description of Related Art
FIG. 13 is a circuit diagram showing a conventional discharge lamp lighting device, and in the figure, 1 is a DC power source such as a battery, 2 is a booster circuit for boosting the voltage of the DC power source 1, and in the booster circuit 2, 2a is a primary winding of a transformer connected to the DC power source 1, 2b is a secondary winding for outputting the boosted voltage, 2c is a switching element connected to the DC power source 1 and the primary winding 2a, 2b is a diode connected to the secondary winding 2b, and 2e is a capacitor. Further, 3 is an earth line, and 4 is a current detection resistor for detecting the output current of the booster circuit 2.
5 is a voltage detection circuit for detecting the voltage Va boosted to the minus side by the booster circuit 2, and in the voltage detection circuit 5, VDD is a 5 V power source, and 5a and 5b are resistors. 6 is an inverter circuit, in which 6a-6d are switching elements. 7 is a control unit for controlling the switching element 2c and a driver 8 which is used for driving the inverter circuit 6. 9 is a startup discharge circuit, in which 9a is a transformer, 9b is a discharge tube switch, and 9c is a capacitor. Further, 10 is a discharge lamp, in which 10a and 10b are electrodes.
FIG. 14 is a circuit diagram showing the details of the conventional control unit, and the control unit 7 in FIG. 13 includes control circuits 7a, 7b, and 7c, an interface 7d (hereinafter abbreviated as I/F 7d), and a microcomputer 7e in FIG. 14. Further, lines A to E in FIG. 13 are connected to A to E in FIG. 14.
In the figure, the control circuit 7a includes an operational amplifier B1, the power source VDD, resistors R1 to R3, and a diode D2, the control circuit 7b includes a comparator A2, the power source VDD, a power source VCC (8 V power source), resistors r3 to r7, and capacitors C1 and C2, and the control circuit 7c includes a diode D1, an operational amplifier A3, a resistor r8, and a capacitor C3.
Now, the operation of the discharge lamp lighting device as configured above will now be described below.
In FIG. 13, when the voltage of the DC power source 1 is input to the booster circuit 2 and simultaneously the control unit 7 starts to operate, the control unit 7 outputs a pulse signal having a certain frequency and a certain duty ratio value to the gate of the switching element 2c, thereby to turn on and off the switching element 2c. Further, the control unit 7 also sends a signal to the driver circuit 8. Whereupon, the driver circuit 8 sends a signal to the gates of the switching elements 6a and 6d to turn on and off the switching elements 6a and 6d. The duty ratio of the pulse signal sent to the gate of the switching element 2c can be varied by the control of the control unit 7.
During the period over which the switching element 2c is ON, the current from the DC power source 1 is supplied to the primary winding 2a and electromagnetic energy is accumulated in the primary winding 2a. Whereupon, an induced counter-electromotive force occurs in the secondary winding 2b, but a reverse bias is applied to the diode 2d, and no current flows in the secondary side of the booster circuit 2. During the period over which the switching element 2c is OFF, an induced counter-electromotive force occurs in the secondary winding 2b, a forward bias is applied to the diode 2d, and a loop is formed by the secondary winding 2b, diode 2d, and the capacitor 2e, so that the electromagnetic energy accumulated in the primary winding 2a during the ON period is accumulated in the capacitor 2e as electrostatic energy through the diode 2d. A voltage Va corresponding to that occurs in the secondary side of the booster circuit 2. ON-OFF of the switching element 2c is repeated by the pulse waveform from the control unit 7, and the voltage Va is gradually boosted to the minus side.
The voltage Va is input to the voltage detection circuit 5, and it is divided by the resistors 5a to 5d and input to the control unit 7. The control unit 7 continues to output to the gate of the switching element 2c a pulse signal of a fixed cycle, which has a predetermined value, for instance, a certain duty ratio value, and which is held when the voltage Va reaches xe2x88x92480 V. To hold xe2x88x92480 V of the voltage Va until the discharge lamp 10 starts to light, the duty ratio is controlled to be a small value of the order of 10 to 50% in the control circuit 7a in FIG. 14. At this point, the output voltage of the operational amplifier B1 lowers, and the diode D2 is electrically conducted and draws out the electric charge of the capacitor C1 to drop the duty ratio to about 10 to 50%. Further, the diode D1 is not conductive immediately before and after the startup of lighting. In addition, since the switching elements 6a and 6d are ON, substantially the voltage Va is applied between both electrodes of the discharge lamp 10.
After the elapse of some time since the voltage Va has reached xe2x88x92480 V, when the voltage difference between both electrodes of the discharge tube switch 9b of the startup discharge circuit 9, or the voltage difference of the capacitor 9c reaches, for instance, about 400 V, the discharge tube switch 9b is turned ON and a current flows through the primary winding of the transformer 9a to generate a high-voltage pulse of about 20 kV in the secondary winding, and the high-voltage pulse is applied to the discharge lamp 10 to cause breakdown between both electrodes of the discharge lamp 10, and a current flows through the discharge lamp 10, which starts to light.
Whereupon, the voltage Va of the electrode 10b of the discharge lamp 10 rapidly rises from xe2x88x92480 V, and the voltage of the electrode 10a becomes a value obtained by multiplying the current flowing through the discharge lamp 10 by the value of the current detection resistor 4. The rapid increase in the voltage Va is detected by the voltage detection circuit 5, and the divided value of the voltage Va is sent to the control unit 7. At this point, the control unit 7 detects the success of the startup discharge of the discharge lamp 10, and continues to turn ON the switching elements 6a and 6d until about several tens msec after the discharge lamp 10 starts to light. With the rapid increase in the value of the voltage Va just after the breakdown of the discharge lamp 10, the electrostatic energy accumulated in the capacitor 2e is supplied as a current to the discharge lamp 10 for about several tens to several hundreds xcexcsec (discharge growth period). Thereafter, by the ON-OFF operation of the switching element 2c, electrostatic energy is again supplied to the capacitor 2e, and a current is also supplied to the discharge lamp 10. Right after the breakdown, the output of the operational amplifier B1 also increases as the voltage Va increases, the diode D2 is not conducted, and the capacitor C1 starts recharged, and thereafter the duty ratio is determined under the control of the control circuit 7c. 
The switching elements 6a and 6d are in the ON state for about several tens msec after the lighting startup of the discharge lamp 10, applying a DC voltage to the discharge lamp 10 to stabilize the discharge, and thereafter a signal is sent from the control unit 7 to an input terminal of the driver circuit 8, and a signal is then output from the driver circuit 8 for turning ON the switching elements 6b and 6c, thereby causing a current of the reverse direction to flow through the discharge lamp 10 for about several tens msec. Thereafter, a signal is output from the driver circuit 8 for alternately turning ON and OFF the switching elements 6a and 6d and the switching elements 6b and 6c to light the discharge lamp 10 in an AC condition, allowing transition of the discharge lamp 10 to a stable steady period.
In the DC lighting period for several tens msec after the lighting startup and the subsequent AC lighting period as described above, the value of the current supplied to the discharge lamp 10 is determined based on the signal from the current detection resistor 4 and the voltage detection circuit 5, and the duty ratio for turning ON and OFF the switching element 2c is controlled by the control circuit 7c to continuously light the discharge lamp 10, so that the voltage Va between both electrodes of the discharge lamp 10 and the current pattern, as shown in FIG. 15, are provided as predetermined. FIG. 15 is the voltage vs. current pattern that satisfies, for instance, maximum current of 2.6 A, maximum power of 75 W, and power of 34 W during the stable steady period. The voltage Vb is the voltage between the electrodes of the discharge lamp 10 in the stable steady period.
FIG. 16 shows a diagrammatic illustration of the current I1 flowing through the discharge lamp 10 during the discharge growth period immediately after the breakdown, the current I2 supplied from the capacitor 2e to the discharge lamp 10, and the current I3 flowing from the secondary winding 2b to the diode 2d, for the case that the discharge lamp 10 is powered off after lighted for a sufficiently long time and soon lighted again. The current I3 is supplied to the capacitor 2e and the discharge lamp 10, but, if the rise of the current I3 is delayed, the current flowing through the discharge lamp 10 becomes less at about the end of the discharge growth period, that is, at the end of the supply of a current from the capacitor 2e to discharge lamp 10 is terminated. For this reason, as shown in FIG. 17A, the discharge lamp 10 easily causes lighting failure at this point of time. If the discharge lamp 10 is lighted after shut off for a sufficiently long time, the current flowing through the discharge lamp 10 is more than the above case, so that lighting failure is difficult to be caused at about the end of the discharge growth period.
To increase the current flowing through the discharge lamp 10, what should be done is only to increase the current I3 during the discharge growth period. However, it is difficult for the control circuit 7 to increase the current I3 by increasing the duty ratio from a smaller value of about 10-50% just before the lighting startup to a larger value, for instance, of about 70-90% in the very short discharge growth period. The reason for this is because the response time of control for changing the duty ratio in the control circuit 7 based on the signal input from the voltage detection circuit 5 is shorter later than the discharge growth period. The voltage input from the voltage detection circuit 5 to a control circuit 7a is input through the operational amplifier B1 and a CR circuit formed of resistors r3 and r4 and a capacitor C1 to a comparator A2, which determines the duty ratio. By the voltage input to the comparator A2, a rectangular waveform having a certain duty ratio is formed, and sent to the gate of the switching element 2c. The capacitor C1 functions to protect the comparator A2 from surge. The time constant of the combination of the operational amplifier B1 and the CR circuit is larger than the discharge growth period, and thus, such control to make the duty ratio a larger value of 70-90% within the very short discharge growth period of about several tens to several hundreds xcexcsec is not possible.
As an example of a conventional discharge lamp lighting device based on the control system as described above, for instance, there is the one described in Japanese Patent Publication No. 2875129.
Since the conventional discharge lamp lighting device is constructed as above, it is difficult to determine the optimum duty ratio of the rectangular waveform sent to the switching element 2c for variations in the output voltage and current of the booster circuit 2, within the very short time of the discharge growth period of several tens to several hundreds xcexcsec just after the breakdown of the discharge lamp 10, and there is a problem that the secondary side output current of the booster circuit 2 cannot be promptly increased just after the breakdown of the discharge lamp 10, and that after the electrostatic energy accumulated in the capacitor 2e is supplied to the discharge lamp 10 as a current, a period appears during which the current supplied to the discharge lamp 10 rapidly decreases, and at that instant, lighting failure easily occurs in the discharge lamp 10.
This invention was accomplished to solve the above problem, and its object is to obtain a discharge lamp lighting device in which the output current of the booster circuit is increased during the discharge growth period after the breakdown of the discharge lamp to eliminate the lighting failure of the discharge lamp.
The discharge lamp lighting device according to this invention comprises a first control circuit for outputting a pulse modulated by pulse width modulation to the switching element of a booster circuit, and a second control circuit for controlling the pulse output from the first control circuit to the switching element in such a manner as to intermittently enable or disable the passage thereof, in order that the DC voltage detected by the voltage detection circuit becomes a predetermined voltage.
In accordance with this invention, the first control circuit may be constructed to carry out a pulse width modulation so as to hold the duty ratio of the pulse output immediately before and after the lighting startup of the discharge lamp to a larger value.
The discharge lamp lighting device related to this invention may further comprise a third control circuit, which determines the lighting startup of the discharge lamp according to the DC voltage detected by the voltage detection circuit, and controls the pulse width modulation by the first control circuit to vary the duty ratio of the output pulse from the larger value to a smaller value during the period of about several tens to several hundreds xcexcsec after the lighting startup of the discharge lamp.